The present invention relates to a multipath distortion eliminating filter which is mounted on an FM receiver to eliminate multipath distortion occurring in reception waves.
The present application claims priority from Japanese Patent Application No. 2003-207869, the disclosure of which is incorporated herein by reference.
Among problems of importance in FM radio broadcasts is interference that results from multipath distortion of the reception waves. Multipath distortion is the phenomenon that an FM reception wave signal, which should basically have a constant amplitude, varies in amplitude because of mutual interference between a plurality of incoming waves having different phases and different field intensities due to multiple wave propagation. In particular, FM receivers mounted on mobile units, such as a car radio, sometimes encounter multipath distortion with sharp fluctuations in amplitude since the state of reception varies with movement. Multipath distortion can cause pulsed noise in FM demodulation signals, contributing to a deterioration in reproduction sound quality.
Conventionally, mobile FM receivers such as a car radio have exercised such controls as ARC (Automatic Reception Control) in order to reduce noise included in the reproduction sound demodulated. In the methods of reducing noise through ARC control and the like, however, the noise suppression has been achieved at the cost of sound quality of some sort, including the stereophonic feel of the demodulated sound. These methods have thus been far from achieving substantial elimination of the multipath distortion.
Now, with the speed up of digital signal processing technologies in recent years, attention is being given to digital FM receivers in which FM reception waves downconverted into intermediate frequency signals are converted into digital signals for digitalized signal processing at the subsequent stages, including wave detection. In such digitalized FM receivers, multipath distortion can be eliminated through the use of adaptive digital filters that have characteristics inverse to the transfer functions of the transmission paths from broadcast stations to the receivers.
FIG. 1 shows an example of the adaptive digital filter for eliminating multipath distortion, which is made of an FIR type filter. Tap coefficients Km of this filter are updated according to the algorithm called CMA (Constant Modulus Algorithm). More specifically, adaptive processing is exercised in consideration of the characteristic of FM signals that the amplitude should basically be constant. Here, the tap coefficients Km are updated and converged so as to minimize an error err between the envelope (amplitude) of the output signal past the filter and a reference value, whereby a filter characteristic for eliminating multipath distortion is provided.
For mobile FM receivers, the reception waves incoming in a multiplex fashion can cause slight deviations in frequency due to vehicle movement. In such a state of reception, components of amplitude variation so-called Doppler fading can occur in the reception waves. With Doppler fading, components of variations as relatively slow as around several hertz to ten-odd hertz are superimposed on the multipath-based sharp amplitude variations of the reception waves.
When reception waves containing such distortion components ascribable to Doppler fading are subjected to the adaptive processing of the filters according to the conventional CMA method described above, however, the filters might follow the Doppler fading and fail to reduce the error err to zero steadily. The adaptive processing of the filters has thus become unstable sometimes.